Add datasette.add_background_task() with supervised launch after startup

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_cli_serve_get.py

@@ -52,6 +52,56 @@ def test_serve_with_get(tmp_path_factory):
     pm.unregister(to_unregister)
 
 
+def test_serve_with_get_does_not_launch_background_tasks(tmp_path_factory):
+    # Per decision #3 in plans/first-request/04-core-plan.md, --get must
+    # never launch background tasks, even though its TestClient request
+    # flows through the full ASGI stack (including the AsgiRunOnFirstRequest
+    # fallback that would otherwise launch them). The plugin's startup hook
+    # itself still runs (registration happens) - only the launch is
+    # suppressed, so the sentinel file the background task would write must
+    # never appear.
+    plugins_dir = tmp_path_factory.mktemp("plugins_for_get_background_tasks")
+    sentinel = plugins_dir / "sentinel.txt"
+    (plugins_dir / "bg_task_for_get.py").write_text(
+        textwrap.dedent(
+            """
+        from datasette import hookimpl
+
+        @hookimpl
+        def startup(datasette):
+            async def inner():
+                async def task(datasette):
+                    with open("{sentinel}", "w") as fp:
+                        fp.write("ran")
+
+                datasette.add_background_task(task, name="get-sentinel-task")
+
+            return inner
+    """.format(sentinel=str(sentinel)),
+        ),
+        "utf-8",
+    )
+    runner = CliRunner()
+    result = runner.invoke(
+        cli,
+        [
+            "serve",
+            "--memory",
+            "--plugins-dir",
+            str(plugins_dir),
+            "--get",
+            "/_memory/-/query.json?sql=select+1",
+        ],
+    )
+    assert result.exit_code == 0, result.output
+    assert not sentinel.exists()
+
+    to_unregister = [
+        p for p in pm.get_plugins() if p.__name__ == "bg_task_for_get.py"
+    ][0]
+    pm.unregister(to_unregister)
+
+
 def test_serve_with_get_headers():
     runner = CliRunner()
     result = runner.invoke(